Recently, mobile communications terminals, electronic appliances, etc. having various functions and high performance have been getting required to be smaller and lighter, so that their electronic parts are arranged in a narrower space at higher density, with their speeds increasing. Accordingly, among circuits and parts, electromagnetic wave noises, particularly high-frequency noises of several hundred MHz to several GHz have become serious problems. To suppress such near-field electromagnetic wave noises, various noise suppression films have been proposed and put into practical use.
Many of such noise suppression films contain magnetic materials and/or conductive materials. For example, JP 2010-153542 A discloses an electromagnetic wave noise suppression film comprising a substrate, a conductive layer formed by a conductive coating material containing particles, flakes or thin wires of metals such as Cu, or carbon, and a magnetic layer formed by a magnetic coating material containing soft-magnetic materials such as ferrite, Sendust, Permalloy, etc. JP 2006-278433 A discloses a composite film for suppressing electromagnetic wave noises, which is obtained by laminating two or more calendered sheets each comprising soft-magnetic powder such as amorphous flakes having a composition of Febal—Cu1—Si12.5—Nb3—Cr1—B12 (atomic %), for example, and a resin, and further calendering the resultant laminate for integration. However, any of the noise suppression films disclosed in JP 2010-153542 A and JP 2006-278433 A does not have sufficient capability of absorbing near-field noises, is difficult to be made thinner because it contains magnetic materials and/or conductive materials blended in the resin, and suffers a high production cost.
JP 2006-279912 A discloses a sputtered thin film of AlO, CoAlO, CoSiO, etc., as a thin film for suppressing near-field electromagnetic wave noises generated in a quasi-microwave band, which has surface resistance controlled to 10-1000 Ω/square matching to the characteristic impedance Z (377Ω) of free space, to have a reflection coefficient (S11) of −10 dB or less and a noise suppression effect (ΔPloss/Pin) of 0.5 or more. However, this thin film for suppressing near-field electromagnetic wave noises does not have sufficient electromagnetic wave absorbability.
JP 2008-53383 A discloses a radiowave-absorbing and shielding film having excellent heat dissipation characteristics, which comprises a graphite film having different thermal conductivities in plane and thickness directions, and a soft-magnetic layer formed on the graphite film, which contains soft-magnetic materials such as Fe, Co, FeSi, FeNi, FeCo, FeSiAl, FeCrSi, FeBSiC, etc., ferrite such as Mn—Zn ferrite, Ba—Fe ferrite, Ni—Zn ferrite, etc., or carbon particles. However, this radiowave-absorbing and shielding film does not have sufficient electromagnetic wave absorbability.
JP 05-226873 A discloses in Example 1 a radiowave absorber obtained by vapor-depositing nickel in a thickness of 12 nm on a 50-μm-thick polyimide film, heating it at 200° C. for 1 hour in air, and then laminating the heat-treated, nickel-vapor-deposited films via an adhesive. However, JP 05-226873 A is completely silent about the problems that (1) when a thin Ni film formed on a polyethylene terephthalate film by a vapor deposition method has surface resistance of several tens of Ω/square, it has excellent absorbability of near-field electromagnetic wave noises, but it is extremely difficult to form such a thin Ni film precisely, and actually formed thin Ni films have largely varying surface resistances, and that (2) the surface resistance of such thin Ni film is subject to large change with time, taking a long period of time until the surface resistance is completely stabilized, and the change with time of the surface resistance differs depending on ambient conditions (temperature, humidity, etc.) during that period.
Accordingly, JP 05-226873 A does not have a purpose of reducing the unevenness and change with time of electromagnetic wave absorbability, thereby stably having excellent electromagnetic wave absorbability. Therefore, heat treatment conditions in JP 05-226873 A are as wide as 50° C. to 400° C. and 30 minutes to 5 hours, 200° C. for 1 hour in Example 1. Further, JP 05-226873 A does not take into consideration the heat shrinkage of a thin Ni film by a heat treatment. Accordingly, JP 05-226873 A lists as substrates for the thin Ni film, plastic sheets such as polyimide, polyethylene terephthalate (PET), polyphenylene sulfide, polyvinyl chloride, etc., and metals such as brass, copper, iron, stainless steel, aluminum, etc., using a polyimide film, which is a heat-resistant resin free from heat shrinkage at heat treatment temperatures, in Example 1.
JP 2006-295101 A discloses a noise-suppressing member comprising a support and a thin nickel film formed on the support, the thin nickel film having an average thickness of 2-100 nm, which meets the condition of 0.5≦log R1−log R0≦3, wherein R1 represents volume resistivity (Ω·cm) converted from the measured value of surface resistance, and R0 represents the volume resistivity (∩·cm) of nickel. Nickel forms fine clusters in the thin film. However, JP 2006-295101 A is completely silent about a heat treatment.
JP 08-59867 A discloses a transparent conductive film comprising a transparent conductive layer made of metals and/or metal oxides such as gold, silver, copper, indium oxides, tin oxides, indium oxide/tin oxide mixtures, etc. formed on at least one surface of a transparent polymer film substrate. JP 08-59867 A describes that after the transparent conductive layer is formed, an annealing treatment is preferably conducted at a temperature of about 120° C. to 200° C. for about 1-30 minutes. However, the transparent conductive layer heat-treated in JP 08-59867 A is not an electromagnetic-wave-absorbing layer, and the transparent conductive layer shown in Example is only ITO (metal oxide).